linux-kernel - Re: [PATCH] dma-direct: Set SG_DMA

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Date: Mon, 6 May 2024 18:19:06 +0200
From: Christoph Hellwig <hch@....de>
To: "T.J. Mercier" <tjmercier@...gle.com>
Cc: Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>, isaacmanjarres@...gle.com,
	Catalin Marinas <catalin.marinas@....com>, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-direct: Set SG_DMA_SWIOTLB flag for dma-direct

On Mon, May 06, 2024 at 09:10:40AM -0700, T.J. Mercier wrote:
> I want to reject mapping a dma_buf for a device if any of the pages
> that back the buffer require SWIOTLB. AFAICT there's no way to know
> whether SWIOTLB is used until after calling dma_map_sg, so afterwards
> I'm trying to check.

You should not check, you simply must handle it by doing the proper
DMA API based ownership management.